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Hardware Information

ECU Specifications

  • Manufacturer: Matsushita/Panasonic
  • Model: Honda PGM/FI
  • Internal Diagnostics: OBD0, no external communication, diagnostic trouble codes only
  • PROM: 27C256 Compatible 32K of program code divided into two 16K sections - Manual and Automatic trans.
  • Fuel Control: Sequential, Multi-port. 6 independently controlled fuel injector drivers
  • Ignition Control: 6 independently controlled coil pack drivers
  • Load Detection: Speed Density (MAP)


  • Termed “ECU” (engine control module) when equipped with a manual transmission, “PCM” (powertrain control module) when equipped with an automatic transmission. A/T control daughterboard is built into ECU.
  • Automatic transmission control unit is integrated within the ECU on it's own separate daughter board. It communicates with the ECU and shares a few of the same sensors. One such communication is A/T shift timing reduction which briefly reduces timing during shifting for smoother shifts.
  • Analog/Digital: 10 Bit A/D Converter. Five Direct A/D inputs for high speed conversion of MAP, TPS, etc. Four A/D Multiplexers IC inputs comprising 32 inputs.
  • Proprietary LSIC Input/Output Controller on E Clock through C000-C01F. Secondary I/O Controller, NSX Only E000-E01F
  • Hardware backup mode to handle internal faults within the ECU - The processor can be physically removed and the engine will still run!
  • Limp home program code to handle single or multiple sensor faults
  • A/C Clutch Control. Adjust idle and delays compressor engagement for fuel enrichment. Disengages clutch under high load.
  • Fuel Cut-off Control

Microprocessor Specifications

MCU Block Diagram
  • Manufacturer: Hitachi H8/532
  • Series: H8/500
  • Model Number: HD6435328CP
  • Package: 84pin PLCC
  • Address Size: 16bit
  • Clock Speed: 10mhz
  • Interrupt Vector Address Space: (0000-007F)
  • Internal ROM: 32K Byte (0000-7FFF)
  • Internal RAM: 1K Byte (FB80-FF7F)
  • Memory Mapped Port Registers: (FF80-FFFF)
  • Processor Mode: Mode2, Expanded Minimum Mode with Internal ROM Enabled. Provisions for changing mode on board.
  • Registers: 8, 16 bit. R0-R7. R6=frame pointer (not used), R7=stack pointer

Inputs and Outputs

ECU Pin Outs

Monitored Inputs (Self Diagnosis / CEL)

These input signals are monitored by the ECU's self diagnostic routine. Any detected failure in these sensors will set a Check Engine Light.

  • Code 1 - Oxygen Sensor Signal Left - 0-3v Amplified A/D
  • Code 2 - Oxygen Sensor Signal Right - 0-3v Amplified A/D
  • Code 3 - Map Sensor Electrical - 0-5v A/D
  • Code 4 - Crank Sensor 1 - Inductive Pickup
  • Code 54 - Crank Sensor 2 - Inductive Pickup
  • Code 6 - Engine Coolant Temp Sensor - 0-5volt A/D
  • Code 7 - Throttle Position Sensor - 0-5volt A/D
  • Code 9 - Cylinder Sensor 1 - Inductive Pickup
  • Code 59 - Cylinder Sensor 2 - Inductive Pickup
  • Code 10 - Intake Air Temp Sensor - 0-5volt A/D
  • Code 11 - Idle Mixture Adjuster - 0-5volt A/D
  • Code 12 - EGR Lift Sensor - 0-5volt A/D
  • Code 13 - Barometric Pressure Sensor - 0-5volt A/D
  • Code 17 - Vehicle Speed Sensor - Magnetic Reed Switch Input
  • Code 18 - Ignition Timing Adjuster - 0-5volt A/D
  • Code 23 - Knock Sensor Left - Piezo Electric Input
  • Code 53 - Knock Sensor Right - Piezo Electric Input
  • Code 35 - TCS STB Signal - 0-5v A/D
  • Code 36 - TCS TCFC Signal - 0-5v A/D

Unmonitored Inputs

  • Alternator FR Signal
  • Battery Voltage (0-14volts)
  • Starter Signal (Switch)
  • Air Conditioning Signal (Switch)
  • A/T Shift Position or M/T Nuetral (Switch)
  • Power Steering Pressure (Switch)
  • Brake Signal (Switch)
  • Service Check Connector (Switch)
  • M/T Clutch Switch (Switch)

Monitored Outputs (Self Diagnosis / CEL)

These output signals are internally monitored by the ECU's self diagnostic routine. Any detected failure in these outputs will set a Check Engine Light.

  • Code 5 - Map Sensor Signal - Software Monitoring
  • Code 14 - 0-12v Linear IACV Control - Direct
  • Code 15 - 6 Channel Independent Coil Over Plug Ignition - Ignition Output Module
  • Code 16 - 6 Channel Independent Fuel Injector Drivers - Direct
  • Code 41 - Oxygen Sensor Heater Left - 12v Continuity
  • Code 42 - Oxygen Sensor Heater Right - 12v Continuity
  • Code 43 - Fuel Supply Left - Software Monitored
  • Code 44 - Fuel Supply Right - Software Monitored
  • Code 45 - Fuel Metering Left - Software Monitored
  • Code 46 - Fuel Metering Right - Software Monitored

Unmonitored Outputs

  • Direct, Check Engine Light Bulb
  • Direct, 2 Channel VIS Butterfly Control Solenoid
  • Direct, FPR Solenoid
  • Direct, PAIR Valve (Type I only)
  • Direct, EVAP Solenoid
  • Direct, EGR Solenoid Duty Control (pulsed output)
  • Relay, Reverse Lockout (6spd Only)
  • Relay, Main - Fuel Pump (multiple outputs)
  • Relay, Air Conditioning Clutch
  • Tachometer Output Signal
  • Traction Control System (multiple outputs)

Emissions Options by Region


  • 9.6:1 Compression
  • Premium Unleaded Fuel
  • Oxygen Sensors
  • Catalyst
  • Knock Sensors
  • EGR
  • EVAP Purge
  • PAIR
  • Injector Test


  • 9.6:1 Compression
  • Premium Unleaded Fuel
  • Oxygen Sensors
  • Catalyst
  • EVAP Purge
  • Knock Sensors
  • EGR
  • PAIR


  • 9.6:1 Compression
  • Premium Unleaded Fuel
  • Oxygen Sensors
  • Catalyst
  • EVAP Purge
  • EGR
  • PAIR

OZDM (Austrailian) (KQ)

  • 9.0:1 Compression
  • Regular Unleaded Fuel
  • Oxygen Sensors
  • Catalyst
  • EVAP


  • 9.0:1 Compression
  • Regular Leaded Fuel
  • EVAP Purge
  • EGR

Africa & Various (KT)

  • 9.0:1 Compression
  • Regular Leaded Fuel
  • EGR

Board Number and Year-to-Year Revisions

There were 3 major changes to the ECU's during the 1991-1995 model run:

  • Gen "A", 1991-1992 Model Years, MCU Code "28", Board Number E-2281A
  • Gen "B", 1993 Model Year, MCU Code "36", Board Number E-2413A
  • Gen "C", 1993-1995 Model Years, MCU Code "3E", Board Number E-2413A

Interchange of external ROM files from the same generation is possible. For instance, a Gen A PY3-A52 ECU can be upgraded with a ROM file from a PY3-A55 ECU which contains many various revisions and updates to program code. A Gen C PY3-A57 Type I ECU can be upgraded with a ROM file from a PX9-A67 ECU for Type II functionality.

Hardware Revisions

There are 2 board versions - 1991-1992 E2281A boards and 1993-1995 E-2413A boards. The newer E2413A board is a lighter color and there are slight changes to component layout. The most notable inclusion is a 5 Pin serial communications header "CN400", which replaces the individual TX and RX test pads of the older ECU. A quick way to tell if you have a 1993-1995 (gen B or C) ECU is to check the top cover. It will have a cut-out with a plastic window on the top cover to view an on board LED. It lights whenever a critical internal problem with the ECU is present and for adjusting idle when in diagnostic mode. It also blinks quickly each time the ignition is turned on.

Software Revisions

There are 3 (incompatible) versions of the program code within the MCU. The last two digits of the part number on the MCU identify the code - YEAMHT532Z28 in this example. This code is the identifier of the revision of the internal coding in the ROM. The coding changes with each revision, with the most drastic taking place between the Gen A and Gen B Ecu's. The Gen B & C ECU's use short format addressing for RAM address 'FCxx - instead of addressing 'FC10, 'FC is dropped and only '10 is required. This saves significant coding space and drastically speeds up operation of the program, hence why the later 93+ generation ECUs are more refined. It appears that there may have been a slight mistake in the 93 MCU coding that required an internal coding change for the 94-95 model years.

ECU Part Number Breakdown

Example: 37820-PY3-A52: 1991-'92, TypeI, USDM, Automatic

37820-PY3-A52 -- ECU Part Number

37820: Honda's part number for any engine computer for any car.

37820-PY3-A52 -- Engine Code

PY3: Type I Engine Code
PX9: Type II Engine Code
PR7: NSX Engine Code

37820-PY3-A52 -- Region Code

A: USDM Federal Emmissions
L: USDM California Emmissions
C: CDM Canadian
G/E/F/S: EDM European
9/J: JDM Japanese
Y: Gulf Spec
Q: Austrailian Spec

37820-PY3-A52 -- Transmission & TCS Type

0: Manual Transmission, Type I or II, without TCS
1: Manual Transmission, Type II with TCS
5: Automatic Transmission, Type I or II, without TCS
6: Automatic Transmission, Type II, with TCS,
7: Automatic Transmission, Type I, without TCS, Revised shift maps

37820-PY3-A52 -- Revision Code

This number goes up with each software revision, the later model cars usually use a higher number

Known Part Numbers

Gen A ECU'S (1991-1992)

37820-PY3-A52: USDM Type I, Automatic
37820-PY3-A53: USDM Type I, Automatic
37820-PY3-A54: USDM Type I, Automatic
37820-PY3-A55: USDM Type I, Automatic
37820-PY3-A02: USDM Type I, Manual
37820-Py3-A03: USDM Type I, Manual
37820-PY3-C52: Canadian, Type I Automatic
37820-PY3-L52: CALI, Type I, Automatic
37820-PY3-G52: EDM, Type I, Automatic
37820-PY3-J03: JDM, Type I, Automatic (note center digit doesn't follow A/T pattern above)
37820-PY3-Y52: Gulf Spec, Type I, Automatic

Gen B ECU's (1993)

37820-PY3-A06: USDM, Type I, Manual
37820-PY3-A57: USDM, Type I, Automatic
37820-PY3-A58: USDM, Type I, Automatic
37820-PX9-A04: USDM, Type II, Manual, NON-TCS
37820-PX9-A15: USDM, Type II, Manual, TCS
37820-PX9-A55: USDM, Type II, Automatic, NON-TCS

Gen C ECU's (1994-1995)

37820-PY3-A72: USDM, Type I, Automatic
37820-PX9-A57: USDM, Type II, Automatic, NON-TCS
37820-PX9-A67: USDM, Type II, Automatic, TCS
37820-PX9-A68: USDM, Type II, Automatic, TCS
37820-PX9-L67: CALI, Type II, Automatic, TCS
37820-PX9-A17: USDM, Type II, Manual, TCS

Hardware Options

R422 Automatic/Manual Transmission


This resistor, R422 controls address line A14 of the external ROM. When R422 is present the A14 address line is pulled high and the upper 16K of the ROM, which is programmed for Automatic Transmissions, is selected. With its absence A14 is pulled low and it uses the lower 16K of the ROM which is programmed for Manual Transmissions. This resistor can be replaced with a switch and a custom dual bank ROM can be programmed to run two separate programs. To convert an Automatic ECU to a Manual ECU simply cut R422!

R411 California/Federal Emissions


This resistor, R411 pulls a multiplexed analog input on IC416 high. It's presence selects Federal emissions. It's absence selects California emissions. There is an internal program switch that can disable checking for this resistor altogether and lock the program in Federal mode.

R409 Traction Control

TCS R409

R409 (in blue) selects whether traction control is supported via the ECU. When supported, the red area of the board is populated with various components to convert traction control signals. There is an internal program switch that can disable checking for this resistor altogether and lock the program in non-TCS mode.

Schematics and Datasheets

Click for full size

Large Integrated Circuits (IC's)

  • IC401: Hitachi 6435328CP10; H8/532 MCU
    • The Processor: AKA the MCU/CPU/Microprocessor/etc. This H8/500 based processor contains 32KB of embedded masked ROM and 1KB of static RAM.
      • Hitachi H8/532 MCU Programming manual contains information on assembly level commands of the processor.
      • Hitachi H8/532 MCU Hardware manual contains information on hardware, layout and interface of the processor.

  • IC404: Matsushita 27C256 ROM Chip
    • The "Chip": Mask ROM (not reprogrammable). Contains all of the variables, fuel and ignition tables and some program code.

  • IC402: Matsushita Proprietary LSIC
    • Due to its proprietary nature, no datasheet is available from Matsushita. This mysterious LSIC chip controls the fuel injectors, ignition timing and various inputs and outputs. It works in conjunction with the MCU and is memory mapped. It runs on a slower E clock signal from the MCU.

  • IC501: Matsushita Proprietary Processor
    • Due to its proprietary nature, no datasheet is available from Matsushita. A custom ASIC of some sort, this processor controls the backup function of the ECU and is called into operation anytime the MCU experiences a hard fault - a **checksum error, internal fault or short circuit within the ECU. It batch fires the injectors and ignition coils and features a 4000RPM rev limit. You could completely remove the processor from the ECU and the engine would still run on this chip.

Miscellaneous IC's

These are standard logic gate IC's

  • IC420, IC418, IC417, IC416: HD14051B; Hitachi Single Ended Analog Multiplexer
  • IC412, IC419: HD74HC14; Hitachi Hex Schmidtt Trigger Inverters
  • IC605: Hitachi HD14069UBP; Hex Inverter
  • IC435: Hitachi HD74HC00P; Logic Gate Quad 2 Input NAND
  • IC431: Hitachi HD14011B; Quad 2 Input NAND
  • IC802: Hitachi HD14025BP; Triple 3 Input NOR Gate
  • IC803, IC603: Matsushita AN6913 Dual Comparator
  • IC407: Hitachi HD74HC139; Dual 2 to 4 Line Decoders
  • IC101, IC601, IC804: JRC 2904DX; Dual OP-Amp

Serial Communications Header


Schematic Diagrams

Address Line Schematic
A/D Input Schematic

I drew two schematic diagrams of the ECU. One was for address and chip select signals to the chip and the other for the A/D direct and multiplexed inputs. The A/D schematic is incomplete because I found the information I needed to complete the disassembly within the program. ~sr5guy

Forum Link Code

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[URL=http://www.acuralegendwiki.org/index.php/G2:_FAQ:_ECU]LegendWiki - G2: FAQ: ECU[/URL]


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